1. Field of the Invention
The present invention relates to an image forming apparatus of, for example, a copier, a printer or a facsimile apparatus or the like.
2. Description of the Related Art
In a related art, a transferring system image forming apparatus of a copier, a printer, a facsimile or the like using a transferring type electrophotography system is constituted by a photosensitive member which is an image bearing member generally of a rotary drum type, a charging apparatus for charging the photosensitive member uniformly at predetermined polarity and potential (charging step), an exposing apparatus as information writing means for forming an electrostatic latent image to the charged photosensitive member (exposing step), a developing apparatus for visualizing the electrostatic latent image formed on the photosensitive member by a toner which is a developer (exposing step), a transferring apparatus for transferring the toner image from a face of the photosensitive member to a transferring material of paper or the like (transferring step), a cleaning apparatus for cleaning the face of the photosensitive member by removing the toner remaining on the photosensitive member after the transferring step although an amount thereof is small (cleaning step), and a fixing apparatus for fixing the toner image on the transferring material (fixing step) and the photosensitive member is repeatedly applied with an electrophotography process (charging, exposing, developing, transferring, cleaning) to form an image.
Inside of the cleaning apparatus is provided with a waste toner collecting container for containing the toner remaining on the photosensitive member removed from the face of the photosensitive member by the cleaning apparatus. Therefore, it is necessary to make the container large-sized to constitute an apparatus having long service life, which is disadvantageous in view of small-sized formation of the apparatus.
Hence, there is an image forming apparatus of a cleanerless system abolishing the cleaning apparatus having the waste toner collecting container and removing and collecting the remaining transferring toner on the photosensitive member after the transferring step from above the photosensitive member by xe2x80x9cdeveloping and cleaningxe2x80x9d in a developing apparatus.
The developing and cleaning is a method of disposing of the remaining transferring toner on the photosensitive member after transferring by charging the photosensitive member at a developing step at a succeeding step and thereafter, that is, successively, exposing the photosensitive member to form an electrostatic latent image, and collecting the remaining transferring toner present on a portion of the face of the photosensitive member (non-image portion) which is not to be developed by the toner by fog removing bias (a fog removing potential difference Vback which is a potential difference between direct current voltage applied to the developing apparatus and surface potential of the photosensitive member) in a procedure of the developing step of the electrostatic latent image.
According to the method, the remaining transferring toner is collected by the developing apparatus and reused in developing the electrostatic latent image at the succeeding step and thereafter and therefore, the waste toner can be eliminated and trouble of maintenance can be reduced and is advantageous in small-sized formation of the image forming apparatus by eliminating the waste toner container.
Meanwhile, in recent years, the charging means is switched to a corona charger, particularly, a roller charging system using a charging roller as a contact charging member is preferably used in view of stability of charging. According to the roller charging, a conductive elastic roller (charging roller) is brought into press contact with a charged body and the charged body is charged by applying voltage thereto.
With regard to the charging system, as disclosed in JP-A-63-149669, an AC charging system for applying voltage constituted by superposing an AC component having peak to peak voltage of 2xc3x97Vth or higher to DC voltage in correspondence with desired surface potential Vd of the charged body to a contact charging member has been proposed and reduced into practice. By the effect of smoothing potential by AC, charging can be made uniform more than a DC charging system and potential of the charged body is substantially converged to Vd which is a center of peaks of AC voltage.
According to the image forming apparatus of a cleanerless system for removing and collecting the remaining transferring toner on the photosensitive member after the transferring step by developing and cleaning in the developing apparatus, in the case of using the contact charging apparatus as the apparatus of charging the photosensitive member, when the remaining transferring toner on the photosensitive member passes a charging portion which is a contact nip portion of the photosensitive member and the contact charging apparatus, particularly a toner in the remaining transferring toner in which charge polarity is inverted to a polarity reverse to normal polarity is adhered to the contact charge apparatus and the contact charge apparatus is contaminated with the toner more than allowable to cause a charge failure.
The cause is derived from the fact the toner as the developer is mixed with a toner in which the charge polarity is inherently inverted to a polarity reverse to the normal polarity, or even a toner having the normal polarity of the charge polarity in which the charge polarity is inverted by being influenced by transferring bias or exfoliation discharge or removed of electricity and having a small charge amount, although an amount thereof is small.
That is, the remaining transferring toner is mixed with the toner having the normal polarity of the charge polarity, the inverted toner having reverse polarity and the toner having the small charge amount and the inverted toner or the toner having the small charge amount is liable to adhere to the contact charge apparatus when passing the charging portion which is the contact nip portion of the photosensitive member and the contact charge apparatus.
Further, in order to remove and collect the remaining transferring toner on the photosensitive member by developing and cleaning of the developing apparatus, it is necessary that the charge polarity of the remaining transferring toner on the photosensitive member carried to the developing portion by passing the charging portion is the normal polarity and the charge amount is the charge amount of the toner capable of developing the electrostatic latent image of the photosensitive member by the developing apparatus. The inverted toner or the toner having the impertinent charge amount cannot be removed and collected from the photosensitive member to the developing apparatus to cause a failed image.
Hence, in JP-A-2001-215798, the applicant has proposed an image forming apparatus provided with uniform remaining transferring toner forming means (first charge providing means) downstream from a transferring portion and provided with a developer charge amount controlling means) (second charge providing means) upstream from charging means for charging the photosensitive member further downstream therefrom.
According to the image forming apparatus, the uniform remaining transferring forming means is means for eliminating a pattern of a remaining toner image in a shape of a pattern on an image bearing member carried from the transferring portion to the developer charge amount controlling means by dispersing to distribute the remaining toner pattern at a face of the image bearing member. Specifically, the pattern of the image of the remaining developer is scratched to destroy or disturbed by abrading the face of the image bearing member by an abrading member to disperse to distribute the developer on the face of the image bearing member.
By providing the image uniform remaining transferring toner forming means, a total normal polarity charging processing is always sufficiently carried out for the remaining transferring toner by the developer charging amount controlling means applied with a successive normal polarity and the remaining transferring toner can effectively be prevented from adhering to the charging means. Further, by erasing the pattern of the image of the remaining toner, a ghost image of the pattern of the image of the remaining toner can be prevented from being brought about.
That is, in the case of absence of the uniform remaining developer image forming means, when transferability of the image of the developer is poor, by, for example, an image of the developer in a vertical line pattern, environment, sheet kind (transferring sheet) or secondary color, the image of the remaining transferring toner in the shape of the pattern on the image bearing member is increased and the image of the remaining transferring toner is carried as it is to the developer charging amount controlling means and developer concentrates on a portion of the developer charging amount controlling means, thereby, there is brought about a phenomenon (toner charge failure phenomenon) in which the charge amount of the remaining developer cannot be controlled at the portion of the developer charging amount controlling means. As a result, there is brought about contamination of the charge memberxe2x86x92charge failurexe2x86x92fogged image formation, or the pattern of the image of the remaining toner remains and the ghost image is brought about.
By providing the uniform remaining toner image forming means, as described above, in the remaining transferring toner image in the shape of the pattern above the image bearing member carried from the transferring portion to the developer charging amount controlling means, even when an amount of the developer is large, the developer is dispersed to distribute at the face of the image bearing member and the patternless formation is produced and therefore, the developer does not concentrate on the portion of the developer charging amount controlling means, the processing of charging a total of the remaining transferring developer into the normal polarity by the developer charging amount controlling means is carried out always sufficiently and the remaining transferring developer is effectively prevented from adhering to the charging means. The ghost image of the remaining developer image pattern is strictly prevented from being brought about.
Further, simultaneously with charging the photosensitive member to predetermined potential by the charging means, the charge amount of the remaining transferring toner charged to the normal polarity by the developer charge amount controlling means is controlled to a pertinent charge amount capable of developing the electrostatic latent image of the photosensitive drum by the developing apparatus. As a result, the remaining transferring toner at the developing apparatus is efficiently collected and an excellent image without having a ghost or the charge failure is provided.
However, when the image is formed continuously by a pattern having a high printing ratio, an amount of the toner accumulated to the uniform remaining transferring toner forming means or the developer charge amount controlling means is increased. Thereby, lowering of charge capacity and deterioration of function are respectively brought about and the patternless formation of the remaining transferring toner and charge to the remaining transferring toner become insufficient to bring about adherence of the toner to the charging means.
Hence, even after continuously printing the pattern of the high printing ratio by a number of sheets, in order to maintain the functions of the uniform remaining transferring toner forming means and the developer charge amount controlling means, it is necessary to periodically discharge the toner accumulated on the uniform remaining transferring toner image forming means and the developer charge amount controlling means. For that purpose, the discharging operation is carried out between sheets or in initial operation when a power source is made ON or when printing operation is finished. In the discharging operation (discharging mode), a bias for positively discharging the accumulated toner is applied. For example, it is known that it is effective to apply a bias having a polarity reverse to that of voltage applied in printing operation or make the bias ON/OFF at short intervals.
Thereby, even in the case of forming an image continuously by a pattern having a high printing ratio, even when an amount of accumulating the toner to the uniform remaining transferring toner forming means or the developer charge amount controlling means is increased, by discharging the accumulated toner by discharging operation at each time, the uniform remaining transferring toner forming means or the developer charge amount controlling means is always refreshed to prevent a deterioration of the charging capacity and function. Thereby, the patternless formation of the remaining transferring toner and charging to the remaining transferring toner can sufficiently be carried out and the toner can be prevented from adhering to the charging means.
However, the following problem is posed in the discharging operation from the uniform remaining transferring toner image forming means and the developer charging amount controlling means.
There poses a problem that the discharged developer (toner) is adhered to the charging member and charge failure and charge nonuniformity are brought about to produce a failure image in forming a successive image.
This is because in the toner discharged from the uniform remaining transferring toner image forming means and the developer charge amount controlling means, other than the toner having the normal charge polarity, there is the reverse toner having the reverse polarity of the charge polarity and the toner having the small charge amount although the toner is provided with the normal polarity and the charge voltage (AC+DC) in the printing operation stays to be applied, the reverse toner or the toner having the small charge amount is adhered to the charging member when passing the nip portion of the contact charge member and the photosensitive member.
Further, the amount of discharging the toner is larger than an amount of the remaining transferring toner in forming a normal image and an amount thereof adhered to the charging member is increased more than normal. By adhering the toner to the charging member, a nonuniformity in charging is brought about, thereby, an image of a fogged image or the like is produced.
Hence, in order to prevent the toner from adhering to the charging member, for example, there is conceivable a method of pertinently switching direct current voltage applied to the charging means when operation of discharging the remaining transferring toner from the uniform remaining transferring toner forming means and operation of discharging the remaining transferring toner from the developer charge amount controlling means are carried out.
However, in order to carry out the method, it is necessary to grasp potential of a photosensitive drum in the discharging operation in details and it is necessary to finely switch voltage applied to the charging member in accordance therewith.
Further, a power source for applying voltage having a polarity reverse that in printing is needed for the charging member to thereby pose a problem of bringing about large-sized formation of a total of the apparatus.
It is an object of the invention to provide an image forming apparatus improving discharge of a developer from a charge providing means for providing charge to a developer remaining on an image bearing member to the image bearing member.
It is other object of the invention to provide an image forming apparatus for constituting surface potential of an image bearing member by a potential suitable for discharging a developer when the developer is discharged from charge providing means.
It is other object of the invention to provide an image forming apparatus for properly setting voltage applied to charging means for an image bearing member for a region constituting a region of discharging a developer of the image bearing member.
It is other object of the invention to provide an image forming apparatus improving discharge of a developer respectively from first and second charge providing means for providing charge to a developer remaining on an image bearing member to the image bearing member.
It is other object of the invention to provide an image forming apparatus for preventing a developer from adhering to charging means from a region of discharging the developer.
It is other object of the invention to provide an image forming apparatus for making a region of discharging a developer from first charge providing means to an image bearing member and a region of discharging the developer from second charge providing means to the image bearing member differ from each other.
It is other object of the invention to provide an image forming apparatus suitable for a cleanerless system which is not provided with an exclusive cleaner.
Further object and characteristic of the invention will become apparent further by reading the following detailed description in reference to the attaching drawings.